WO 2004/031092 Al JI i J II I í I; I ll I ll I I II I I 1 I I I I I JÍ
Eurasian patent (A, A2, B and, KG, KZ, MD, RU, TJ, TM), For two-le.ver codes and oiher abbrevimions. refer the ilie "Cuid- European paient (AT. BE, BG, CH, CY, CZ, DE, D, EE, anee Noies on Codes and Abbr viations" appearing ai e begm- ES, Fl, FR, GB, GR HU, IE, IT, LU, MC, NU PT, RO, no ofeach regular issue ojihe PCT Gazene SE, SI, SK, TR), OAPI patent (BF, BJ, CF, CG, CI, CM, GA, GN, GQ, GW, ML, MR, E. SN, TD, TG). Publishcd: - wüh intemaiionai searck repon
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METHOD AND PLANT FOR THE MANUFACTURE OF CEMENT CLIN ER
DESCRIPTION OF THE INVENTION The present invention relates to a method for the manufacture of clinker or cement slag by which method the raw cement flour is preheated and burned in a plant comprising a cyclonic preheater and an oven. The invention relates specifically to a method for reducing the emission of S02, CO and volatile organic compounds (hereinafter referred to as VOC) of such a plant. The invention also relates to a plant for carrying out the method. Plants of the aforementioned type for the manufacture of cement clinker are well known from the literature. The emission of S02, CO and VOC from such kiln plants for the manufacture of cement clinker emanate mainly from the raw materials that are used as described in more detail in the following text. The heating of the raw meal in the cyclonic preheater is carried out by direct contact with hot exhaust gases according to the principle of countercurrent flow, by which the S02 and CO formed and the expelled VOCs are captured immediately by the gas stream of escape, and therefore leave the preheater
Ref .: 161930 2
cyclonic together with the exhaust gas stream in the form of emission. For different reasons, the three types of emission to the atmosphere are undesirable. The cement raw material frequently contains minerals such as pyrite and marcasite. The sulfur in the pyrite (FeS2) becomes the cyclonic preheater at temperatures around 525 ° C, causing S02 to form. Therefore, measurements made in a cement plant in operation have shown that virtually all the sulfur contained in the raw meal feed will still be present in the raw meal when it leaves the first cyclonic stage at a temperature of approximately 370 ° C, while the sulfur content in the raw meal leaving the second cyclonic stage at a temperature of approximately 550 ° C will be approximately half. Thus, in the cement plant in question about half of the sulfur contained in the raw material will escape from the preheater in the form of S02 as a result of the oxidation process which takes place in the second cyclonic stage. A known method to reduce the level of S02 involves the application of an absorber in the form of CaO, Ca (OH) 2 or other basic components in the cyclone preheater at a location (seen in the direction of flow of the exhaust gases), after S02 is formed in such a way that the S02 can bind to the raw material in the form of sulfite which 3
is transformed into sulphate in a subsequent process step. A significant disadvantage of this known method is that it will usually be necessary to apply an amount of excess absorbent, thereby making it a relatively expensive method. Likewise, the raw material of the cement will frequently contain organic carbon which is expelled substantially from the raw flour in the form of CO and VOC during the preheating process in the cyclonic preheater and which is discharged in unburned form together with the gas stream escape. This is confirmed by studies that indicate that certain types of VOCs are expelled substantially in a relatively narrow temperature range. A type of VOC is then expelled substantially in a temperature range that varies from 300 to 500 ° C, while another type is expelled substantially in a temperature range that varies from 450 to 650 ° C. Other additional types of VOCs are expelled in higher temperature ranges. In a traditional cyclonic preheater the aforementioned temperature range will typically take place in the first and second cyclonic stages, respectively, the second and third cyclonic stages, depending on whether the cyclone preheater is a four-stage or five-stage unit and also a little dependent on the efficiency of the other elements in 4
the furnace system. Several methods are known for the subsequent treatment of exhaust gases to remove VOCs from the exhaust gases. A known method comprises the steps, that the exhaust gases of the preheater are reheated in a heat exchange unit, that the VOCs are burned while simultaneously adding fuel, and that the exhaust gases are subsequently cooled in the exchange unit. of heat. From the point of view of energy consumption this is not an optimal solution, and the apparatus to carry out the method also involves quite substantial investment costs. Furthermore, from the Danish patent application No. PA 2001 00009 a method is known by which raw flour is removed from the preheater and from the heater in a separate heating unit to form S02 and to expel VOC. According to the known method, the formed SO2 is subsequently contacted with an absorbent, the expelled VOCs are burned, and the raw flour is reintroduced into the cyclonic preheater. The disadvantage of this known method is mainly that the energy consumption will be relatively high. An object of the present invention is to provide a method as well as a plant for the manufacture of cement clinker by means of which an economical and effective reduction of S02, CO and VOC can be achieved without any significant impact on the efficiency level of the cyclonic preheater. This is achieved by a method of the type mentioned in the introduction characterized in that at least a part of the raw flour is extracted from the cyclonic preheater, the raw flour is introduced in a separate unit in which it is given a retention time under oxidizing conditions provided by means of a gas stream to form S02 and to expel organic carbon, the SO2 formed and the organic carbon expelled are subsequently discharged from the separate unit entrained in the gas stream for further treatment in a subsequent process step, and the Raw flour is reintroduced into the cyclonic preheater. With this, an effective reduction in the emission of VOC, CO, as well as S02 is obtained without needing the use of additional energy for heating. By giving a retention time to the raw flour extracted and partially preheated under oxidizing conditions separately from the cyclonic preheater, the result is that the sulfur will oxidize to SO2 and organic coal will be expelled from the raw flour, in such a way that the S02 well formed and organic carbon well 6
Expelled can be entrained in a relatively small separate gas stream and subjected to subsequent treatment in the optimum manner. As will be described in more detail in the following textc, the studies carried out by the applicant have surprisingly indicated that significant oxidation of sulfur to S02 and some expulsion of organic carbon will take place even when the temperatures remain constant and below those of the location in the cyclonic preheater where otherwise most of the S02 release is occurring. Studies have also shown that the speed at which these processes take place depends on the temperature and that the speed will increase in tandem with the rise in temperatures. The plant for carrying out the method according to the invention is characterized in that it comprises means for extracting at least a part of the crude flour from the cyclonic preheater, separate means for giving this raw meal a retention time under oxidation conditions and ensuring therefore the oxidation, by means of a gas stream, of sulfur contained in this crude flour for the formation of S02 and for the expulsion of organic carbon, means for discharging the formed SO2 and the organic carbon expelled from the separate unit entrained in the gas stream for additional treatment in a step of 7
subsequent process, and means to re-introduce the raw meal into the cyclonic preheater. Additional characteristic features of the plant can be appreciated from the detailed description provided in the following text. It is preferred that all of the raw flour is extracted from the cyclonic preheater for oxidation in a separate unit. So far, conventional wisdom has argued that when the raw material contains sulfur components, S02 will form in a relatively small temperature range around 525 ° C. However, the studies mentioned above and described in greater detail in the following text have indicated quite surprisingly that significant oxidation of sulfur to S02 will occur even at lower temperatures if only the time necessary for the process is allocated. Studies have thus shown that the formation of S02 can occur even at a temperature of 350 ° C, and therefore, according to the invention, raw flour can be extracted from the cyclonic preheater at a temperature ranging from 350 ° C. and 525 ° C. In order to limit the retention time necessary for the raw meal extracted in the separate unit and hence its capacity, it is preferred that the raw meal be used for the production of the raw meal.
Remove from the cyclonic preheater at a temperature in the range of 400 ° C and 500 ° C. The studies carried out have indicated that at temperatures higher than 525 ° C, S02 will form so rapidly that virtually all of the sulfur will have been converted to S02 before the raw flour is extracted from the preheater. In principle, the raw meal can be given any retention time in the separate unit which is necessary to achieve the desired S02 formation at the temperature in question. In actual practice, the temperature of the extracted raw meal will be the main determinant for the duration of the retention time, which is necessary. According to the invention, the retention time in the separate unit can be selected randomly, but will often be advantageously in the range of 10 to 200 seconds. However, it is preferred to apply a maximum limit of 100 seconds. The temperature in the separate unit can be kept substantially constant during the oxidation process, but it can also vary, for example, by regulating the temperature of the gas stream introduced in the separate unit. If it is desired to increase the oxidation rate, the temperature in the separate unit can then be raised by introducing a hotter gas stream.
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In principle, organic carbon is expelled from raw flour throughout the temperature range in the cyclonic preheater, which in the case of raw flour varies from a temperature lower than 100 ° C in the upper part of the cyclonic preheater until a temperature near 830 ° C at the bottom of the cyclonic preheater. Therefore, the method according to the invention will only have the ability to eject a portion of the total amount of organic carbon. The temperature at which the raw meal will be extracted will therefore depend mainly on the temperature at which the maximum reduction of the S02 level is achieved. The aforementioned studies in the text have indicated variations in the pattern to expel different types of organic carbon. Therefore, before the implementation of the method according to the invention in a given cement plant, it will be advantageous to carry out specific investigations of the raw material used in order to accurately determine its content of different types of organic carbon and determine additionally how they are expelled as a function of temperature. In some cases, which are consistent with the reduction of SO2, it is therefore preferred that the raw flour is extracted from the cyclonic preheater at a temperature of less than 450 ° C.
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In principle, the oxidation of the raw flour extracted in the separate unit can be done in any suitable way, however a small stream of oxygen-containing gas must be directed through the compartment for the oxidation of sulfur and organic carbon and for the removal of S02 and for the removal of S02 and expelled organic carbon. Studies have indicated that the optimum percentage of oxygen to remove S02 is approximately 5 percent. The separate unit can be configured in any suitable way. The separate unit may comprise any type of receptacle or transport mechanism for bulk materials, which will be able to provide a sufficient retention time for the raw meal and ensure sufficient mixing of the raw meal and the gas stream containing oxygen . For example, the separate unit can be configured as a rotating drum in which the raw flour extracted and the gas stream containing oxygen are introduced through inlets located at each end of the rotating drum, pass through the rotary drum in flow at countercurrent and are also discharged from the opposite ends. In addition, it is desirable that the unit or plant includes means to ensure that the raw meal after its removal from the unit will be physically located at, or may be directed to the level that is necessary to be re-introduced at the designated location in the cyclone preheater.
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The formed S02 that is discharged from the separate unit entrained in the gas stream can be treated separately, for example, in a wet scrubber of a known operating principle where S02 in the reaction with CaCO3 and ¾0 will be transformed into gypsum in the form of CaS04 »2H20, and from which cleaned gas can be released into the environment. The necessary CaCO3 may be contained in the powder carried along the separation unit or may be supplied in the form of fresh raw flour. Due to the fact that the gas stream through the separated oxidation unit is small relative to the gas stream through the cyclonic preheater, the wet scrubber for this purpose can also be relatively small. Water consumption will also be relatively small. The gypsum generated in the wet scrubber can be used for sale in the cement grinding plant in substitution of some of the ordinary gypsum. In this way a significant amount of sulfur can be diverted from the kiln system of the cement plant, thereby reducing the problems that frequently occur with respect to clogging and obstruction in the kiln system. The formed S02 which is discharged from the separate unit entrained in the gas stream may alternatively be introduced into the cyclone preheater at a site where a sufficient amount of absorbent is present at the site.
form of CaO and / or other basic components, which will typically be at the lower end of the cyclonic preheater. In plants where the cyclonic preheater includes a calciner, it is preferred that the formed S02 is introduced into that calciner. The expelled organic carbon can be burned separately or alternatively it can be reintroduced to a location in the cyclonic preheater where the temperature is at least 700 ° C, which will typically be at the lower end of the preheater. In plants where the cyclonic preheater includes a calciner, it is preferred that the expelled organic carbon be introduced into that calciner. In principle, raw flour extracted and oxidized separately can be reintroduced into the cyclone preheater anywhere. However, it should be introduced preferably immediately after the point of extraction of raw meal, seen in the flow direction of the raw meal. In other words, it is preferred that the raw oxidized flour be separately introduced into a cyclonic preheater in the first cyclone stage after the cyclone stage from which it was extracted. In the following, the invention will be described in more detail with reference to the figures, which are in the form of a diagram, and in which:
Figure 1 shows a graphic representation of the formation of S02 as a function of time at different temperatures, Figure 2 shows a first example of a plant according to the invention, and Figure 3 shows a second example of a plant in accordance with the invention. with the invention Figure 1 shows graphs a - d for the formation of S02 as a function of time at temperatures of 350, 375, 400 and 500 ° C. The illustrated graphs are a direct result of a series of tests which have been carried out by the applicant. The tests have been carried out in a fixed bed reactor according to the method described below. A sample of material is pre-heated to a desired temperature in an inert hot gas consisting of pure N2. The hot exhaust gas from the inert heating process was mixed with 02 in order to oxidize any sulfur evaporated in S02. Following the inert heating for a period of 240 seconds, 02 was added to the N2 stream before the stationary bed. This procedure was followed to be certain that there would be no significant oxidation of the material sample during the relatively slow heating process and that the supply of 02 would take place rapidly in relation to the chemical reaction in such a way that
that the reaction rate could be studied at a given temperature and independently of the heating process. The content of S02 as a function of time was measured through the test procedure. In figure 1 part of the graphs where t is less than 0 show the formation of S02 which occurs in relation to the heating of the material in the inert gas while the graphs for t greater than 0 show the formation of S02 when the temperature is kept constant and when 02 is supplied. It is clearly noted from the figure that the formation of S02 is a very slow process during the inert heating process to the point where t equals 0 and that subsequently, when supplied 02 when t equals 0 the formation speed is much faster. In particular the graphs b, c and d show that the conversion of sulfide to S02 takes place predominantly in the initial period of 100 seconds from which the graphs rise, presumably tending towards a maximum conversion in which, with respect to the material that is Used during the test, it will be approximately 30, 50 and 70 percent, respectively, at the respective temperatures of 375, 400 and 500 ° C. Therefore, outside of these three temperatures, the immediate conclusion would be that the optimum method could involve the extraction of the raw material at a temperature of approximately 500 ° C since the 15th degree
High conversion is achieved at this temperature. However, it should be noted that in relation to heating up to a level of 500 ° C in the inert gas, a fairly significant amount of SO2 will be formed which in actual practice will not be transferred to the separate unit. Therefore, the optimum temperature to extract the material in question seems to be in the range of 400 ° C to 500 ° C. In Figures 2 and 3 two examples according to the invention are observed. Both plants shown comprise a cyclonic preheater 1, a rotary kiln 5 and a clinker cooler 7. The cyclonic preheater 1 comprises four stages of cyclones a-d, a calciner 3 and a separation cyclone 4. The cyclonic preheater 1 can comprise less or more than the four indicated cyclones. The raw flour from a raw flour grinding plant is introduced into the cyclonic preheater through one of several inlets 9 and is preheated in a countercurrent arrangement with exhaust gases from where it is subsequently separated from the cyclone preheater in cyclone d and directed to calciner 3 in which it is calcined. From the bottom outlet of the separation cyclone 4, the calcined raw meal is then directed through the duct 8 to the rotary kiln 5 in which it is burned to form cement clinker which is then cooled in the clinker cooler 7. Exhaust gases from the rotary kiln 5 and 16
calciner 3 are extracted through cyclone 4 and subsequently through the cyclonic preheater by means of a fan. Tertiary air is introduced from the clinker cooler 7 through the conduit 11 in the calciner 3. According to the invention at least a part of the raw flour is extracted from the cyclonic preheater 1 with a view by subjecting it to oxidation in a separate unit 21 in wherein the raw meal is introduced through the duct 15 in order that the separate oxidation of the raw meal has some significant effect on the formation of S02 and the expulsion of organic carbon, the raw meal must of course be extracted from the preheater before that most of the sulfur content has been transformed into SO2 and / or before the organic carbon has been expelled from the material. In cases where it is only desirable to carry out the separate oxidation of some of the raw meal, it can be extracted from the flow of the raw meal from the bottom outlet of the selected cyclone stage through, for example, a sluice gate 13. The separate unit 21 shown in Figure 2 and 3 comprises a rotating drum 21. A gas stream is introduced through an inlet 27 at one end of the rotating drum and the extracted raw flour is introduced through an inlet. 29 at the opposite end, causing the mixing of the raw flour and the gas stream to take place.
in a countercurrent flow arrangement. The raw flour is extracted from the other end of the rotary drum 21 through an outlet 22 and directed through a conduit 26 and a transport means, if any, back to the cyclonic preheater 1 in which it is reintroduced to through an entrance 28 which is located immediately after the point at which it was extracted, seen in the direction of the raw flour flow. In the embodiment shown in Figure 2 the gas stream containing S02 and the expelled organic carbon leaves an outlet 29 in the rotary drum 21 through a conduit 17 to the calciner 3 in which all the organic coal is burned and S02 is absorbed under optimal temperature conditions by reaction mainly with CaO. In the embodiment shown in Figure 3 the gas stream containing S02 and the expelled organic carbon exits the outlet 29 in the rotary drum 21 through the conduit 17 to a wet scrubber 31 in which it is cleaned in accordance with known principles where the S02 by reaction with CaCO3 and ¾0 will be transformed into gypsum based on the CaSO4 »2H20 form, and from which the cleaned gas can be released to the environment, possibly through a unit 33 to burn CO and VOC. In relation to the implementation of the invention in an existing cement plant it will often be necessary to
Set the temperature in the preheater to a level at which it will allow the raw flour to be extracted at the desired temperatures. This can be done in many ways. If it is desired to lower the temperature at the specific site in the preheater where the raw meal is to be extracted for separate oxidation, it will be possible to introduce, for example, atmospheric air at an appropriate site. However, if it is desired to raise the temperature at the specific extraction point, the feed of raw meal can, for example, be divided and a small amount of raw meal diverted. Also, the temperature can be adjusted by controlling the amount of raw flour that is extracted for the separated oxidation. Another means of regulation consists of a modification or regulation of the separation efficiency of the preheater cyclones. In actual practice it may be necessary to regulate the capacity of the separate unit 21 and in the case of a rotating drum this can be done by changing its rotation speed. An increase in the rotational speed of the rotating drum will result in a reduction in the retention time of the raw meal in the drum, bringing with it a corresponding reduction in the amount of S02 that is formed and the organic coal being expelled. In order to compensate for any such reductions, heat may be provided to the separate unit 21, possibly 19
by introducing a stream of partial flow of hot air from clinker cooler 7 into unit 21 through inlet 27. It is noted that with respect to this date, the best method known to the applicant to carry out the practice said invention is that which is clear from the present description of the invention.